core: prevent self-impersonation

[BeryJu]

closes #6863

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)
• The translation files have been updated (make i18n-extract)

If applicable

• The documentation has been updated
• The documentation has been formatted (make website)

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (44ac944) 92.71% compared to head (423aa64) 92.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6885      +/-   ##
==========================================
- Coverage   92.71%   92.65%   -0.05%     
==========================================
  Files         567      567              
  Lines       27939    27959      +20     
==========================================
+ Hits        25900    25903       +3     
- Misses       2039     2056      +17     

Flag	Coverage Δ
e2e	50.58% <0.00%> (-0.57%)	⬇️
integration	26.27% <0.00%> (-0.01%)	⬇️
unit	89.61% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
authentik/core/api/users.py	92.64% <100.00%> (+0.69%)	⬆️
authentik/core/tests/test_impersonation.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[github-actions]

authentik PR Installation instructions

Instructions for docker-compose

Add the following block to your .env file:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-core-fix-self-impersonation-1694627979-a0d10ad
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

For arm64, use these values:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-core-fix-self-impersonation-1694627979-a0d10ad-arm64
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

Afterwards, run the upgrade commands from the latest release notes.

Instructions for Kubernetes

Add the following block to your values.yml file:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
image:
    repository: ghcr.io/goauthentik/dev-server
    tag: gh-core-fix-self-impersonation-1694627979-a0d10ad

For arm64, use these values:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
image:
    repository: ghcr.io/goauthentik/dev-server
    tag: gh-core-fix-self-impersonation-1694627979-a0d10ad-arm64

Afterwards, run the upgrade commands from the latest release notes.